Outer plates, bumpers and so forth of vehicles such as cars, buses and trains are coated in various color combinations and patterns in respect of functions and designs. In this coating, coat masking tapes are used in order to separate coat colors or protect portions which need not be coated.
As performances required in coat masking tapes, there are required, e.g., that the masking tape can be stuck in a good operability on curved surfaces of coating objects having complicated shapes, that paint lines after coat baking can be formed in a good precision, that coating materials can well adhere to the back of the masking tape, and that the masking tape can be peeled with ease (releasability at low temperature or high temperature).
As base materials for coat masking tapes, crepe paper, polyester film, soft vinyl chloride film, polyolefin resin film and so forth are used. As pressure-sensitive adhesive compositions used to form pressure-sensitive adhesive layers on such base materials for coat masking tapes, known compositions are, e.g., a composition constituted chiefly of natural rubber and added thereto a metal oxide, a tackifier and a vulcanization accelerator, and a composition constituted chiefly of natural rubber and a styrene-butadiene copolymer and added thereto a metal oxide, a tackifier, polyisobutylene, zinc oxide, heat-reactive phenol resin and so forth.
These conventional base materials for coat masking tapes, however, have the following problems. For example, the crepe paper has problems that an impregnating resin used to improve paper quality may exude under conditions of baking a coating material and come to the coating surface, the paper has no follow-up properties to the complicated curved surfaces, and no good paint lines can be formed. The polyester film has a problem that positions at which the masking tape can be applied are limited because of its poor follow-up properties to curved surfaces. The soft vinyl chloride film is advantageous as a base material having overcome the disadvantages of the crepe paper or polyester film, but has a problem that, when the coat masking tape is peeled at a relatively low temperature of from 5.degree. C. to 10.degree. C. after a coating material is coated and baked, the base material tends to break and its fragments scatter to tend to contaminate the coating objects. The polyolefin resin film has a problem that the adhesion of coating materials to the film is so poor that the coating materials may come off or the paint lines are not sharply formed.
Meanwhile, since the above known compositions used to form the pressure-sensitive adhesive layer are constituted chiefly of natural rubber, they have a problem of having an insufficient durability to heat. Hence, the part on which the masking tape is stuck tends to be contaminated after baking, to cause problems that final products have no sufficient stability, masking tapes do not well adhere in the winter or can not be well peeled after baking, and in some cases the pressure-sensitive adhesive layer may be left on the coating object when the tape is peeled.
The present invention aims at solving the above problems in the prior art. Accordingly, an object of the present invention is to provide a coat masking tape base material having superior follow-up properties, promising sharp paint lines and being free of "left-on" when peeled, and a coat masking tape making use of such a base material.